Infections with RSV are one of the major causes of viral lower respiratory tract illness. Protection against RSV may be achieved with an efficient vaccination strategy that induces neutralizing humoral immunity and a Th1-dominant cellular response and that does not predispose to Th2-dominant vaccine-induced exaggerated RSV disease. Based on the knowledge that adenovirus (Ad) gene transfer vectors can be used to evoke robust systemic and mucosal immunity against an immunogen expressed as a transgene and that Ad functions as a potent adjuvant, this proposal focuses on the development of a vaccine against RSV using modified Ad vectors. The Ad modifications include the addition of an RGD motif to the fiber knob, a modification known to enhance infection of antigen presenting cells and enhance Th1 immunity, as well as the incorporation of RSV epitopes into the Ad capsid. To assess if anti-RSV immunity can be elicited even in the presence of anti-human Ad immunity, the vectors will be based on the non-human primate serotype AdC7, against which humans do not have immunity. These modified vectors will be assessed for the ability to induce immunity and protection against RSV in mice, with particular focus on potential vaccine-enhanced RSV lung disease. Two specific aims outline the studies to achieve these goals. Aim 1. To evaluate the hypothesis that an optimized AdC7 vector, which is engineered to increase activation and infection of antigen presenting cells, and which expresses the RSV F protein, as a transgene, will evoke robust protective immunity against RSV without causing vaccine-induced RSV disease and in the context of pre-existing human Ad immunity. Aim 2. To evaluate the hypothesis that modified AdC7 vectors engineered to contain epitopes of the RSV F and G proteins in the capsid hexon protein, a strategy that enables boosting of the immune response with the identical vector, will evoke robust immunity against RSV without predisposing to vaccine-induced RSV lung disease.